Is quantum mechanics complete?
Perhaps not. Adam Becker’s “What is Real?” makes a case for alternative theories to counter the hegemony of the Copenhagen Interpretation.
The world described by quantum mechanics is a strange one. To fully appreciate its strangeness, picture this in your mind: Imagine a ball rolling down a track at a speed of 1 meter per second. Classical physics tells us this: if we can measure the position and velocity of the ball at this time, we can predict the position and velocity of the said ball at different points in the future.
Even if no one is looking at the ball, everyone would agree that the ball has definite properties. If our measurements do not disturb the system, we can measure the properties without changing anything about the ball at all. All that changes is our knowledge. We now know something additional about the world that we previously did not know.
Now, imagine that the ball is shrunk to the size of an electron. The Copenhagen Interpretation of quantum mechanics tells us that the electron does not have any definite properties before the act of measurement. In fact, it exists in a superposition of all possible values until someone comes along to measure it. When that happens, the ‘wavefunction’ of the system collapses, giving rise to a singular definite value. It is not just our knowledge of the world that changes. By choosing to take a measurement, we are changing something deeply fundamental about the external world.
To even describe what the Copenhagen Interpretation of quantum mechanics means will probably require an entire book. In my opinion, the simplest and best analogy one can give is that of Schrodinger’s cat. In this thought experiment, a cat is trapped in a sealed box with a vial of poison and a radioactive source. The radioactive source has a 50/50 chance of decaying. If it decays, a hammer hits the vial, killing the cat in an instant. Otherwise, the cat carries on as usual. However, according to the Copenhagen interpretation, the cat is in a superposition of dead-and-alive states —in everyday language, both dead and alive — before an observer looks and seals the fate of the cat.
If this sounds ridiculous, it is because it is meant to be. Schrodinger devised this thought experiment to point out all the problems with the Copenhagen Interpretation. What counts as an observer? Does a measuring instrument count? Can the cat observe its own demise? The Copenhagen Interpretation will tell you that it was never meant to be applied to macroscopic objects such as cats. The macroscopic world and microscopic world operates by two different sets of laws. If that is the case, where do we know to draw the line?
Critics of the Copenhagen Interpretation will tell you this: if the theory is so strange, it is because it is incomplete. It is incomplete in two ways. Firstly, a complete theory should tell us everything about the system we are observing. We should be able to talk about what is happening in the box even when no one is checking on the status of the cat.
Critics also claim that physicists have not spent enough time working out the philosophical implications of the theory. That assertion is the central thesis of Adam Becker’s “What is Real? The Unfinished Quest for the Meaning of Quantum Physics”. He argues that the Copenhagen Interpretation of quantum mechanics is far from complete. The philosophical questions raised by the theory have not been satisfactorily put to rest by its forebears. Nor has much progress been made in the following decades. The reluctance of the physics community to embrace alternative theories were shaped not only by scientific considerations but also by sociological and political reasons. It is only recently that advancements in other fields such as quantum computing have reopened the avenues for alternative theories to challenge the hegemony of the Copenhagen Interpretation.
The book is divided into three parts. In the first part of the book, Becker highlights the philosophical foundations that underlie the theoretical considerations of quantum mechanics. Unlike modern physicists who are quick to show their disdain for philosophical thought, physicists such as Einstein and Bohr put great emphasis on the philosophical underpinnings of their theories.
It is a well-known fact that Einstein and Bohr disagreed heartily over the completeness of the Copenhagen Interpretation. At the core of their disagreement lay fundamentally different views about what science was supposed to do. For Einstein, a theory should provide “a complete description of any real situation”, without taking external observers into account. Any theory that failed to do so was ‘incomplete’. On the other hand, Bohr stopped short at having his theory describe measurable features and predict measurements. The idea of an external world that exists beyond human perception was not taken into consideration. All that really matters was describing what happens when it comes into contact with an observer via measurement.
In “What is Real?”, Becker frames their disagreement as a standoff between logical positivism and realism. Realism, in its simplest version, postulates an external world that exists beyond our perception, independent of the presence or absence of observers. On the other hand, logical positivism regards experience as indispensable to the formation of knowledge. Central to their argument was the ‘verification theory of meaning’, which held that only statements verifiable empirically were meaningful. For example, statements like, “the table is round” is empirically verifiable, while “the table is beautiful” is simply an expression of emotion. Therefore, if a theory postulates things that cannot be directly observed, they are not to be considered meaningful.
Becker asserts a tentative link between logical positivism and the Copenhagen Interpretation. However, this claim is often challenged by philosophers of science. The subject of Bohr’s philosophical inclinations and the philosophical background of the Copenhagen Interpretation is a complex subject that has spawned a body of scholarly work. As Becker readily admits, Bohr is famously vague in his writings, making it quite impossible to determine his philosophical leanings. Any attempt to describe him as a logical positivist, a Kantian, or even an unrealizing Marxist is bound to meet with some contrary evidence or other. Whatever his views were, one thing is clear: he is not alone in this aspect. Other physicists of his time, whose intellectual upbringing involved a reasonable amount of philosophy, were well-equipped with the cognitive tools to consider and discuss the philosophical underpinnings of their theories.
All these changed in the disruption caused by the Second World War. As the epicenter of physics shifted from Europe to the United States, physics broke ties with its philosophically-minded European roots and embraced the pragmatism of the States. The colossal success of the Manhattan Project secured military funding, ensuring the influx of brilliant young minds into Physics departments. An increase in classroom sizes meant lecturers were not at leisure to challenge the orthodox interpretation of quantum mechanics. Textbooks that avoided discussing the philosophical foundations altogether received praise, for such “philosophically tainted questions” were no longer considered relevant in this new milieu.
In the meantime, the Copenhagen Interpretation was doing very well for itself. Apart from some awkward (and even that qualification depends on your philosophical position) hiccups in explaining the nature of reality, it faced no trouble at all in making predictions that were incredibly successful. In the face of this, the reluctance of physicists to accept alternative theories was understandable. “Shut up and calculate!”— became the silent motto of many physicists, unwilling to jeopardize their careers in search of an alternative. If the Copenhagen Interpretation was not broken, why fix it?
The second part of the book deals with the rise of alternative theories that challenged the dominance of the Copenhagen Interpretation. One notable example is David Bohm’s pilot-wave theory. In his theory, the wavefunction is a physical entity that nudges the particles into their positions. A way to visualize it would be to imagine a tow truck(the pilot wave), towing a defunct car (the particle) along the motorway. Unlike the Copenhagen Interpretation, the wavefunction does not undergo the mysterious collapse when a measurement is conducted. Instead, particles already have definite positions that change along with the wavefunction. This description makes the Bohm’s theory rather similar to classical physics.
Bohm’s theory managed to solve the paradoxes raised by the Copenhagen Interpretation without sacrificing mathematical integrity. The question of the role of the observer no longer applied, because entities had definite states even before the act of measurement. Also, Bohm’s theory applied to all entities, even in the macroscopic world— the question of the division between the quantum and the classical no longer mattered. In short, it was a theory that would appeal to anyone who subscribed to realism.
Bohm’s theory did not immediately attract the attention it deserved. Bohm’s communist sympathies were viewed with suspicion by the authorities, eventually driving him out of the country and cutting off his access to his European and American colleagues. Stripped of the opportunity to give talks in defense of his theory, Bohm cast his work aside and settled down to work on other problems.
The third part of Becker’s “What is Real?” describes the revival of interest in alternative theories of quantum mechanics. The work of John Bell, a physicist attached to CERN, was a game-changer. In a short, but important paper, Bell proved that hidden variables and locality could not be simultaneously preserved without violating a set of constraints, termed ‘Bell’s Inequality’.
Bohm’s pilot wave theory was vindicated in part by Bell’s Inequality. Bohmian mechanics involved hidden variables and was non-local, two things that went against the grain of both the Copenhagen Interpretation and Einstein’s relativity. In other words, particles in Bohmian mechanics have definite positions independent of observation and can be affected by other particles entangled in the same wavefunction. A set of experiments conducted by Clauser and Freedman proved that Bell’s inequality was violated. Although the consequence of Bell’s theorem and its set of experiments are not fully understood, Clauser’s (and later, Aspect’s) experiments paved the way for the return of Bohmian Mechanics.
At the same time, the field of physics was being radically transformed by progress in quantum computing. Quantum computing sparked a revival of interest in the foundations of quantum mechanics, long left untouched by a generation of physicists. Young physicists like Clauser who attempted to question the foundations faced a series of formidable challenges, from opposition from their senior colleagues to difficulties in securing permanent positions. (Bell always advised any young physicist interested in his theorem to find himself a secure position first). Revolutions in quantum computing and cryptography promised important applications for the military. This ensured the second wave of funding after the first wave had petered out with the closing of the Cold War. As a result, it became not only respectable but also lucrative to work on questions concerning fundamental quantum mechanics.
Quantum computers also proved to be a useful tool in vindicating Bohmian Mechanics. With unrivaled processing speeds, quantum computers were put to the task to simulate scenarios based on Bohmian Mechanics. The results were promising— Bohmian Mechanics could account for the results of experiments, most notably the double-slit experiment in which particles exhibited wave-like properties. This, in turn, sparked a renewal of interest in Bohmian Mechanics, most notably in Bohm himself. A wave of papers ensued, developing new derivations of Bohmian mechanics and building defenses against its detractors. In this changing political climate, the shadow of suspicion and doubt has been lifted. Bohmian Mechanics was finally granted the attention it had been denied for nearly three decades.
As Becker concludes in the final chapter of “What is Real?”, the history of science is shaped by both scientific considerations and non-scientific forces. Science, as a human endeavor, is never free from political and social influences. Any attempt to understand its history is not complete without taking these biases into account. To what extent these biases affect the objectivity of science is a subject of debate among philosophers and historians of science. It is also a very real challenge faced by this book, as evident in some critical reviews.
As someone who has barely scratched the surface of the physics, history, and philosophy of quantum mechanics, I find myself incapable of giving anything more than a summary of Becker’s “What is Real?”. All I can say is this: any attempt to construct an overarching narrative of a field that has been developed over a century by the likes of Bohr, Heisenberg, and Pauli (not to mention Bohm and Bell) will naturally overlook certain nuances. And physicists and philosophers alike will continue disagreeing on the claims raised by the book, such as the completeness of the theory. Becker’s narrative is by no means to be taken as the final say. Even so, Adam Becker’s “What is Real?” is a useful read for anyone who wishes to acquire a different perspective of the history of the field.